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MAC0010.1177/0020294014551627Tech Talk (4) Pressure Measurement BasicsTech Talk (4) Pressure Measurement Basics
Themed Paper
Tech Talk: (4) Pressure
Measurement Basics
Measurement and Control
2014, Vol. 47(8) 241­–245
© The Institute of Measurement
and Control 2014
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DOI: 10.1177/0020294014551627
mac.sagepub.com
John E Edwards and David W Otterson
I. Introduction
Accurate and reliable pressure
measurement is a requirement for the
safe operation of most industrial
processes. It is probably the
measurement parameter most applied by
the Instrument Engineer. The object of
pressure measurement is to produce a
dial indication, control operation or a
signal, typically the standard 4–20 mA,
that represents the pressure in a
process.
Pressure measurement is obtained
from the effects of pressure which cause
position movement, change in resistance
or other physical effects which are then
measured. The most common pressure
sensors or primary pressure elements
employ a Bourdon tube, diaphragm,
bellows, force balance or variable
capacitance arrangement. Some other
methods are also outlined later in this
article.
II. Pressure Units and
Terminology
Process pressure is defined as the force
applied to a surface area, for example,
kg/m2. The SI unit for pressure is Pascal
(Pa), but bar is more commonly used for
process measurement. Table 1 shows
the relationships for the more common
pressure units.
Pressure is a relative measurement
defined as either gauge or absolute.
Gauge pressure varies with atmospheric
pressure, which in turn varies with the
altitude above sea level and the weather
conditions. The relationship between
these definitions is shown in
Figure 1.
Absolute pressure pa is the pressure
above a total vacuum, and gauge
pressure pg is the pressure above or
below atmospheric pressure patm giving
pa = pg + patm for all pg where pg is
negative if less than patm
To avoid sign confusion, pressures
below atmospheric pressure are referred
to as pvac giving
pa = patm – pvac for pg < patm
We can see from Table 1 that
1 atm = 14.696 psi = 1.01325 bar which is
equivalent to 0 psig and 0 barg. We can
deduce that 30 psig = 44.696 psia = 3.082 bara and 10 psia = 4.696 psi
vac = −4.696 psig. It is recommended
that absolute pressures are stated as
‘psia’ or ‘bara’ and gauge pressures are
stated as ‘psig’ or ‘barg’ to prevent
confusion.
Gauge pressure is the unit most
encountered, with a good example being
vehicle tyre pressures which are in gauge
pressure. A gauge pressure device will
indicate zero pressure when vented to
atmosphere.
Absolute pressure includes the effect
of atmospheric pressure with the gauge
pressure. An absolute pressure indicator
would indicate atmospheric pressure (not
scale zero) when vented to atmosphere.
perpendicular to the flow direction, while
having little impact on surfaces parallel to
the flow direction. This directional
component of pressure in a moving
(dynamic) fluid is called dynamic
pressure. An instrument facing the flow
direction measures the sum of the static
and dynamic pressures; this
measurement is called the total pressure.
Since dynamic pressure is referenced to
static pressure, it is neither gauge nor
absolute; it is a differential pressure.
While static gauge pressure is of
primary importance in determining net
loads on pipe or vessel walls, dynamic
pressure is used to measure flow rates
and airspeed. Dynamic pressure can be
measured by taking the differential
pressure between instruments parallel
and perpendicular to the flow. Pitot–
static tubes, for example, perform this
measurement on aircraft to determine
airspeed. The presence of the measuring
instrument inevitably acts to divert flow
and create turbulence, so its shape is
critical to accuracy and the calibration
curves are often non-linear.
Dynamic pressure can be expressed as
pd = 0.5 ρ V2
where pd is the dynamic pressure (Pa), ρ
is the density of fluid (kg/m3) and v is the
velocity (m/s).
A. Static and Dynamic Pressure
B. Differential Pressure
Measurement
Static pressure is uniform in all directions,
so pressure measurements are
independent of direction in a stationary
(static) fluid. Flow, however, applies
additional pressure on surfaces
Differential pressure (dp), as the term
implies, is the pressure difference
between two points of measurement.
Typical applications include pressure
drops in ventilation systems, across
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Tech Talk: (4) Pressure Measurement Basics
Table 1. Pressure units conversion
From
To
psi
kg/cm2
bar
mm Hg
atm
psi
1
0.07031
0.06895
51.715
0.06805
kg/cm2
14.223
1
0.9807
735.6
0.98692
bar
14.504
1.0197
1
750.06
1.01972
mm Hg
0.01934
0.00136
0.00133
1
0.00131
atm
14.696
1.0332
1.01325
760
1
Figure 1. Full
Vacuum
Absolute Pressure (pa)
Absolute Pressure (pa)
Gauge Pressure (pg)
-ve
+ve
Vacuum Pressure (pvac)
Pressure
Absolute
Zero
Atmospheric
Pressure (patm)
Figure 2. both vented or pressurised vessels and
for gas pressure measurement on lowpressure vessels where a dp
transmitter with the low-pressure side
vented to atmosphere would give more
accurate results than a pressure
transmitter.
C. Pressure Gauges
primary flow elements such as an orifice
plate, venturi or Pitot tube and across
process equipment such as prime
movers, filters and process columns.
Measurements of differential pressure
are also used to find other quantities by
making use of known formula, for
example, liquid level and density on
Pressure gauges, of the dial type shown
in Figure 2, can be used for test
purposes, pneumatic signals or local
process indication. The pressuremeasuring element can be a Bourdon
tube, diaphragm or bellows which are
available in a wide variety of materials to
satisfy process fluid compatibility.
Process measurement can be in the
range full vacuum to 2000 barg.
Pneumatic signal measurement use
receiver gauges which have measuring
ranges 3–15 psig or 0.2–1 barg.
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Readability and location will determine
the size of gauge specified. Generally,
the larger the gauge diameter, the more
accurate will be the reading as more
graduations can be incorporated.
Manufacturers’ recommendations vary,
but in general, the normal operating
pressure of a gauge should be at around
75% of the scale presuming an adequate
design/overpressure safety margin.
Pressure gauge selection criteria
should include the measurement
accuracy required. The following
guidance is derived from B40.1 and
B40.7 contained in Standard ASME
B40.100. Standard BS EN 837-1:1998
also addresses this issue.
Grade 4A gauges offer the highest
accuracy and are calibrated to ±0.1% of
span over the entire range of the gauge.
The gauges are called laboratory
precision test gauges. These highaccuracy test gauges may be
temperature compensated. They must
be handled carefully in order to retain
accuracy.
Grade 3A gauges are calibrated to an
accuracy of ±0.25% of span over the
entire range of the gauge. The gauges
are called test gauges but are generally
not temperature compensated.
Grade 2A gauges are calibrated to an
accuracy of ±0.5% of span over the
entire range of the gauge. These gauges
are generally used for process pressure
measurement. They are often referred to
as process gauges and are not
temperature compensated.
Grade 1A gauges are calibrated to an
accuracy of ±1% of span over the entire
range of the gauge. These gauges are
high-quality general purpose industrial
gauges.
Grade A gauges are calibrated to an
accuracy of ±1% of span over the middle
half of the scale and ±2% of span over
the first and last quarters of the scale.
These gauges are often referred to as
industrial gauges.
Grade B gauges are calibrated to an
accuracy of ±2% of span over the middle
half of the scale and ±3% of span over
the first and last quarters of the scale.
These gauges are often referred to as
commercial or utility gauges.
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Tech Talk: (4) Pressure Measurement Basics
Note that wide ambient temperature
excursions from that at which a nontemperature-compensated Bourdon
gauge has been calibrated can lead to
significant reading errors, typically ±0.4%
of span for each 10°C change (rising or
falling) from a reference temperature of
20°C.
If pulsation is present in the process,
the maximum operating gauge pressure
should not exceed 50% of the full-scale
range. A safety blow out panel is
normally specified for gas pressure
gauges. Other considerations in gauge
selection include the following:
•• Gauge mounting: direct, rear or front
flange;
•• Orientation of pressure connection;
•• Pressure connection thread type;
gauges of 100 mm diameter and
above would normally have 0.5″- or
15-mm thread connections for
strength and stability;
•• Liquid filling (typically glycerine or
Fluorolube® for oxygen service) –
used where vibration or severe
pulsation is present and for wet
environment including under water
applications;
•• Hermetic sealing – used where
ambient conditions are corrosive,
very dusty humid or wet. Ingress
protection (IP) rating to be specified
accordingly;
•• Case material – typically, brass,
stainless steel, phenolic or
polycarbonate;
•• Oxygen service cleaning;
•• Maximum pressure red pointer;
•• Maximum pressure follower pointer;
•• Alarm electrical contacts;
•• Pulsation dampener (snubber);
•• ‘Pigtail’ – a coiled pipe used to
introduce a condensate cooling
barrier between the process and the
instrument in steam applications;
•• Flow restrictor in-line coupling – for
high-pressure gas applications.
For dirty, viscous process fluids such
as heavy oil or slurries, the measuring
element must be protected from the fluid
using diaphragm seals (Figure 3). The
diaphragm seal may be directly mounted
Figure 3. Figure 4. to the gauge or remote mounted using a
filled capillary connecting tube.
Pressure gauges are high-maintenance
items requiring frequent replacement to
ensure correct service. It is worth
considering the more robust and expensive
pressure transmitter for critical applications.
Programmable digital pressure gauges
(Figure 4) are available, capable of
measuring gauge, absolute and
compound ranges. (A compound
pressure gauge is scaled from full
vacuum through zero pressure up to the
full-scale pressure.)
These instruments allow a selection of
measurement units and are available with
4–20 mA output and alarm switches.
III. Pressure Transmitters
Pressure transmitters are available with
pneumatic or electronic transmission and
can be specified to measure gauge
compound or absolute pressures
(Figure 5). (A compound pressure
transmitter provides a linear output signal
from full vacuum through zero pressure
up to the full-scale pressure.) Sensing
element–measuring principles include
capacitive, piezoelectric and force
balance methods. Measured pressures
up to 2000 barg, with accuracies as low
as ±0.06%, and span turn-downs of up
to 10:1 are available.
Remote seals are used when the
process fluid is corrosive, viscous,
subject to extreme temperature, toxic, in
sanitary applications or tends to collect
and solidify. The seals are available to
suit all standard flange fitting types. A
wide range of remote seal types are
Figure 5. available which allow optimum design for
flanged, wafer, flush mounting or hygienic
applications for use in the food and
pharmaceutical industries (Figure 6).
When a compact pressure transducer
is required, such as on a compressor
lubricating system, a range of miniature
diaphragm sealed devices is available
(Figure 7).
Two-, three- and five-valve manifolds
(valves manufactured as a composite
block) are available, providing safe
pressure transmitter process isolation
and on-line calibration facilities. Smart
communications and signal transmission
capability with 4–20 mA, 4–20 mA/HART,
4–20 mA/FoxCom, FOUNDATION
Fieldbus, Profibus or Low Power 1–5 V
output electronic modules are available.
Certifications can be obtained to meet
hazardous area requirements, for example,
Flameproof or Intrinsically Safe and also
Safety Integrity Level (SIL) standards.
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Tech Talk: (4) Pressure Measurement Basics
Table 2. General guidance on selection
Figure 6. Figure 7. IV. Pressure Switches
A pressure switch is a device capable of
detecting a pressure change, and at a
predetermined level, opening or closing
an electrical or pneumatic contact.
Pressure switches find application in many
areas ranging from engine oil pressure
monitoring and electrohydraulic control
through to the monitoring of process fluid
pressure for alarm and switching duties.
Both pressure and differential pressure
switches are available.
Much of the primary element
technology to be found in pressure
gauges and pressure transmitters can be
found in the construction of pressure
switches. This includes the Bourdon
tube, bellows, diaphragm and solid-state
designs. To these may be added piston
and diaphragm–piston-type pressure
switches.
Faced with a huge choice of
instrument types, the engineer may either
specify the duty and leave the selection
to the manufacturer or stockist or apply
engineering know-how to select the best
Parameter
Bourdon tube
Diaphragm
Piston
Diaphragm/
piston
Solid state
Range
3–1200 barg
Full vacuum
to 10 barg
1–850 barg
Full vacuum to
70 barg
Full vacuum
to 70 barg
Accuracy
±0.5%
±0.5%
±2%
±2%
±0.25%
Cycle rate
≤25 cycles/min
≤25 cycles/
min
≤50 cycles/
min
≤50 cycles/min
>50 cycles/
min
Life cycles
106 cycles
106 cycles
106 cycles
2.5 × 106
cycles
100 × 106
cycles
instrument for the application. Most
ground rules used for the specification of
pressure gauges and transmitters apply
for the specification of pressure switch
wetted parts materials, process
connections, housing materials, IP,
hazardous area and functional safety
certification.
In addition to the above, the engineer
must understand and specify the
requirements for the following:
••
••
••
••
••
••
••
••
••
Operating pressure range;
Design pressure limits;
Accuracy and hysteresis limits;
Dead-band limits;
Cycle speed, number of operations
and life expectancy;
Number of switching points;
Fixed or adjustable switching point/s;
Electrical load rating and
configuration of switch contacts;
Ambient temperature limits.
General guidance in selecting the type
of primary element to be used is shown
in Table 2. The data are extracted from a
manufacturer’s published catalogue1 and
thus may vary between manufacturers.
Current catalogue data should always be
checked.
applied force (pressure) over an area.
Bourdon tubes. Bourdon tubes are
circular-shaped tubes with oval cross
sections (Figure 8). The process pressure
acts on the inside of the tube. The
outward pressure on the oval cross
section forces it to become more
rounded.
Because of the curvature of the tube
ring, the bourdon tube then bends with
the resulting movement being
transmitted to the gauge pointer by
gearing.
Bellows. Bellows or capsule-type
elements are constructed of tubular
membranes that are convoluted around
the circumference (Figure 9). The
membrane is attached at one end to the
source and at the other end to an
indicating device or instrument.
For differential pressure transmitters,
the capsule is constructed with two
diaphragms forming an outer case. The
inter-space is usually filled with a process
compatible fluid. Pressure is applied to
both sides of the diaphragm, and it will
deflect towards the lower pressure. To
provide over-pressurised protection, a
solid plate with matching convolutions is
mounted in the centre of the capsule.
V. Pressure-Sensing Technology
A. Force Collector Types
These types of pressure sensors generally
use a force collector (such as a diaphragm,
piston, bourdon tube or bellows) to
measure strain (or deflection) due to
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Diaphragms. A diaphragm is a circularshaped convoluted membrane that is
attached to the pressure fixture around
the circumference (Figure 10). The
pressure medium is on one side and the
indication medium is on the other.
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Tech Talk: (4) Pressure Measurement Basics
Figure 8. systems (MEMS). Generally, this
technology is considered to provide very
stable readings over time.
Figure 10. Thermal. The changes in thermal
conductivity of a gas due to density
changes are used to measure pressure.
A common example of this type is the
Pirani gauge which is a robust gauge
used for the measurement of the
pressures in vacuum systems.
Figure 9. technologies are mostly applied to low
pressures.
Electromagnetic transducer. The
displacement of a diaphragm is
measured by means of changes in
inductance (reluctance), linear voltage
displacement transducer, Hall effect or
by an eddy current principle.
Piezoelectric transducer. The
piezoelectric effect in certain materials
such as quartz is used to measure the
strain upon the sensing mechanism due
to pressure.
Piezoresistive strain gauge
transducer. The piezoresistive effect is
based on bonded or formed strain
gauges to detect strain due to applied
pressure using various thin-film
technologies. The strain gauges are
connected to form a Wheatstone bridge
circuit to maximise the output of the
sensor and to reduce sensitivity to
errors. This is the most commonly
employed sensing technology for
general purpose pressure
measurement.
Capacitive transducer. A diaphragm
and a pressure cavity are used to create
a variable capacitor to detect strain due
to applied pressure. Common
technologies use metal, ceramic and
silicon diaphragms. Generally, these
Potentiometric transducer. The motion
of a wiper along a resistive mechanism is
used to detect the strain caused by
applied pressure.
B. Other Types
Resonant. The changes in resonant
frequency in a sensing mechanism are
used to measure the stress, or changes in
gas density, caused by applied pressure.
This technology may be used in
conjunction with a force collector, such as
those in the category above. Alternatively,
resonant technology may be employed by
exposing the resonating element itself to
the media, whereby the resonant
frequency is dependent upon the density
of the media. Sensors have been made
out of vibrating wire, vibrating cylinders,
quartz and silicon micro-electromechanical
Ionisation. It measures the flow of
charged gas particles (ions) which varies
due to density changes to measure
pressure. Common examples are the hot
and cold cathode gauges.
VI. Conclusion
The instrument engineer is faced with a
bewildering choice when specifying
pressure-measuring instruments. It is
recommended that the potential vendors
are supplied with full data covering the
physical properties of the process, including
the normal measurement operating range
and extremes, the operating environment,
output signal form, hazardous area and
functional safety requirements. The vendor’s
data sheet should always be crosschecked against the above requirements.
Funding
This research received no specific grant from
any funding agency in the public, commercial
or not-for-profit sectors.
References
1. Selection Guides. Barksdale Control Products, Inc.
2. Edwards JE. Process Measurement & Control
in Practice. 1st ed. Thornaby: P&I Design Ltd,
2011.
3. Morris AS. Principles of Measurement and
Instrumentation. Englewood Cliffs, NJ: Prentice
Hall, 1993.
4. Rangan CS, Sharma GR, Mani VSV.
Instrumentation Devices and Systems. New Delhi,
India: Tata McGraw Hill, 1993.
5. Liptak BG. (ed.). Instrument Engineers Handbook –
Process Measurement and Analysis. 3rd ed.
Radnor, PA: Chilton Book Company, 1995.
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